Without knowing more about what you see I can think of a possible reason.:

Would “occasionally” be about every 3-months? (Jan, April, July, Oct).
ie. When Ofgem changes the price caps so your PAYG tariff changes.

Although the tariff prices change on a set day it may take a few days for your PAYG smart meter to get updated to those new prices.
Meaning that for a number of days it has been taking off the wrong prices for the usage and for the standing charge - so it then needs a balance adjustment to correct the under/over payment for those days.

Hey ​@Lauralala87 

Sorry to hear about this, it would be more unusual to see with pay as you go but there could be a few reasons, including that of what Nukecad has mentioned regarding the price cap changes. If perhaps there’s had been a delay in getting the prices updated, but we can’t know for sure here at the Forum as we don’t have access to accounts.

It would be good to know how often this has happened?

My other thought would be any potential additional winter payments such as the Warm Home Discount, this could also potentially have appeared as an adjustment.

If you can let us know a few more details it may help us, but the Support Team would be able to view any adjustments made on the account, so it may be worth getting in touch with them to double check.